Ocean Monument location requirements haven't changed is quite some time so Amidst will work with 1.12 for finding them.
I suggest that you download Amidst v4.3beta2, run it, select a VANILLA profile for the PCMC version you prefer, and start hitting Ctrl-R. (the link is in my signature)
That will cause Amidst to display the biome map of random seeds. Hint: Place Amidst in it's own folder and create an empty file in that folder named "history.txt".
When Amidst starts up it will detect that file and from then on will create a log of every seed that it displays.
That way, if you hit Ctrl-R too quickly you can find out what the seed was of the previous map and redisplay it.
Ctrl-H gives you the seed input dialog.
I use Notepad++ as my default text editor.
Below is the Amidst map of "worstly" ("1525413704")
Rollback Post to RevisionRollBack
There are no dangerous weapons. There are only dangerous people. R.A. Heinlein
If you aren't part of the solution, then you obviously weren't properly dissolved.